The proposed experiments will investigate how prior knowledge and expectations affect the control of visual attention and processing of visual inputs. Specifically, activity in the cholinergic nuclei of the basal forebrain is hypothesized to signal the level of uncertainty subjects have about upcoming events. Under conditions of low uncertainty, top-down attention-related signals representing subjects' expectations should be emphasized in the analysis of visual information in early cortical areas. On the other hand, when uncertainty is high, there is little useful information in these top-down inputs, and bottom-up stimulus-related signals should be facilitated. The technique of functional magnetic resonance imaging (fMRI) will be employed to measure brain activity in different brain areas while subjects are making visual judgments under conditions of low or high uncertainty. These studies will also identify candidate areas in the parietal and frontal lobes that mediate attention to a location in space versus attention to specific features of visual objects. Finally, the interactions between bottom-up sensory inputs and their modulation by top-down attentional signals will be measured in early visual cortex and correlated with performance on psychophysical tasks. In addition to providing information about how attention is represented in the brain and how it affects processing of visual stimuli, the present experiments should also shed light on the function of the cholinergic projection from the basal forebrain to the cerebral cortex. As this projection is thought to be selectively impaired in Alzheimer's disease, knowledge concerning its function in the healthy brain may be useful for understanding the etiology of this disease and for development of appropriate therapies.